1. Field of the Invention
The invention relates generally to micro-mechanical devices.
2. Background Information
MicroElectroMechanical Systems (MEMS) are miniature devices that combine micro-electronics with micro-mechanics. The MEMS, which typically have a size ranging from less than a millimeter (m, one thousandth of a meter) to more than about a micrometer (μm, one millionth of a meter), may be used individually or as systems to perform a variety of functions. Many commercially important applications of MEMS have already been established in the areas of telecommunications (e.g., microscopic optical switches such as those available from Lucent Technologies), display technology (e.g., digital mirrors such as the Digital Mirror Device available from Texas Instruments), automotive technology (e.g., micro-accelerometers for airbags such as the ADXL micro-accelerometer from Analog Devices), as well as other applications as diverse as, but not limited to, computer memory, drug delivery, and ink jet printing. The number of commercially important applications of MEMS is expected to grow significantly in the foreseeable future.
FIG. 1 shows an exemplary MEMS 100 having a polysilicon cantilever beam 120 that is fixedly attached at an anchor 130 to a polysilicon contact plate 150 that is in turn fixedly attached to a silicon substrate 110. Without limitation, the cantilever 120 may have a length of about 100 micrometers and a width of about 60 micrometers. Also attached to the silicon substrate 110 is a polysilicon contact plate 140 proximate to an end of the cantilever 120. The MEMS 100 may exist substantially in two electromechanical states. As shown, a first state involves a separation distance between the end of the cantilever 120 and the polysilicon contact plate 140. A second state involves sufficient mechanical and electrical contact and coupling between the end of the cantilever 120 and the plate 140. A signal or stimulus may be provided to the MEMS 100 to cause the MEMS 100 to change between the two states.
Two significant problems facing MEMS are frictional wear and stiction. Frictional wear is a common mode of failure and degraded performance for many MEMS. MEMS are often fabricated of silicon by using techniques similar to those used to manufacture integrated circuits. Silicon may wear significantly during operation of the MEMS device. For example, the end of the cantilever 120 may wear against the plate 140. Gears and other engaging MEMS components may experience this problem to an even greater extent.
Stiction is another problem facing MEMS. Stiction is the unintended distortion of adjacent parts as a result of forces such as electrostatic charge build up, surface tension due to a release rinse liquid, or attraction between surface adsorbed water molecules. For example, charge may build up at the end of the cantilever 120 causing the cantilever 120 to unintentionally bend into connection with the contact 140.
One approach to remedy these problems involves coating a MEMS with a self assembled monolayer, as described in the paper, “Self-Assembled Monolayers As Anti-Stiction Coatings For MEMS: Characteristics And Recent Developments”, by Roya Maboudian, Robert Ashurst, and Carlo Carraro and published in Sensors and Actuators (82): 219-223, 2000. FIG. 2 shows a MEMS beam portion 220 coated by a self-assembled monolayer 200 of octadecyltrichlorosilane, (CH3)(CH2)17SiCl3 210. This self-assembled monolayer approach has a number of limitations. One limitation is that the coatings are organic and have a very limited range of properties. The organic coatings do not work well for several surfaces commonly used in MEMS. Another limitation is that the coatings are limited to a single monolayer, and as such do not provide sufficient electrical insulation, or sufficient protection against frictional wear.
Another approach is to use chemical vapor deposition (CVD) to coat a MEMS. An application of a CVD to a microengine was described in the article entitled, “Effect of W coating on micro engine performance,” by authors S. S. Mani et al., reported in the IEEE 38th Annual International Reliability Physics Symposium, in San Jose, Calif., 2000, pages 146-151. However, CVD has several significant limitations that render it generally unsuitable for MEMS. One limitation is that the high deposition temperatures for CVD are often not suitable for MEMS. For example, the temperature for tungsten deposition by CVD is generally not less than about 450° C., and may be even higher (e.g., 650° C.). Differences in coefficients of thermal expansion between the deposition layer and underlying materials, over the large temperature range between deposition temperature and ambient, may create significant material stresses between the deposition layer and underlying materials. Another limitation is that the deposited layers do not have uniform thickness. In particular, CVD primarily deposits on line-of-sight surfaces, which means that bottom surfaces and small void regions receive less, or almost no coating.
Accordingly, prior art methods for depositing layers on MEMS have a number of significant limitations and improved systems and methods for depositing layers on MEMS are needed.